Jul 8, 2016 - cess of prograde angular momentum possibly caused by ... right away a mass/size comparable to that of Phobos ... formed the calculation using a Bulk Silicate Mars .... (b) In the co-accretion scenario, circular and co-planar.
Draft version July 11, 2016 Preprint typeset using LATEX style emulateapj v. 01/23/15
RECONCILING THE ORBITAL AND PHYSICAL PROPERTIES OF THE MARTIAN MOONS T. Ronnet1 , P. Vernazza1 , O. Mousis1 , B. Brugger1 , P. Beck2 , B. Devouard3 , O. Witasse4 , F. Cipriani4
arXiv:1607.02350v1 [astro-ph.EP] 8 Jul 2016
1
Aix Marseille Universit´ e, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France 2 Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France 3 Aix-Marseille Universit´ e, CNRS, IRD, CEREGE UM34, 13545 Aix en Provence, France and 4 European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands Draft version July 11, 2016
ABSTRACT The origin of Phobos and Deimos is still an open question. Currently, none of the three proposed scenarios for their origin (intact capture of two distinct outer solar system small bodies, co-accretion with Mars, and accretion within an impact-generated disk) is able to reconcile their orbital and physical properties. Here, we investigate the expected mineralogical composition and size of the grains from which the moons once accreted assuming they formed within an impact-generated accretion disk. A comparison of our results with the present day spectral properties of the moons allows us to conclude that their building blocks cannot originate from a magma phase, thus preventing their formation in the innermost part of the disk. Instead, gas-to-solid condensation of the building blocks in the outer part of an extended gaseous disk is found as a possible formation mechanism as it does allow reproducing both the spectral and physical properties of the moons. Such a scenario may finally reconcile their orbital and physical properties alleviating the need to invoke an unlikely capture scenario to explain their physical properties. Keywords: planets and satellites: composition, formation, individual (Phobos, Deimos) 1. INTRODUCTION
During the 70s and 80s, dynamicists have demonstrated that the present low eccentricity, low inclinations and prograde orbits of Phobos and Deimos are very unlikely to have been produced following capture (Burns 1978; Pollack et al. 1979), thus favoring a formation of the moons around Mars (Szeto 1983; Cazenave et al. 1980; Goldreich 1963). Despite such early robust evidence against a capture scenario, the fact that the moons share similar physical properties (low albedo, red and featureless VNIR reflectance, low density) with outer main belt D-type asteroids has maintained the capture scenario alive (Fraeman et al. 2012, 2014; Pajola et al. 2013). Whereas the present orbits of the moons are hardly compatible with a capture scenario, they correspond to the expected outcome of an in situ formation scenario either as the result of co-accretion or of a large impact. Co-accretion with Mars appears unlikely because Phobos and Deimos would consist of the same building block materials from which Mars once accreted. Those building blocks would most likely comprise water-poor chondritic meteorites (enstatite chondrites, ordinary chondrites) and/or achondrites (e.g., angrites), which are all suspected to have formed in the inner (≤2.5 AU) solar system, namely interior to the snowline. This assumption is supported by the fact that the bulk composition of Mars can be well reproduced assuming ordinary chondrites (OCs), enstatie chondrites and/or angrites as the main building blocks (Sanloup et al. 1999; Burbine & O’Brien 2004; Fitoussi et al. 2016). Yet, OCs as well as the remaining candidate building blocks (enstatite chondrites, angrites) are spectrally incompatible with the martian moons, even if space weathering effects are taken into account (see panel b in Figure 1). It thus appears from above that accretion from an
impact-generated accretion disk remains as the only plausible mechanism at the origin of the martian moons. As a matter of fact, the large impact theory has received growing attention in recent years (Craddock 2011; Rosenblatt & Charnoz 2012; Canup & Salmon 2014; Citron et al. 2015). This hypothesis is attractive because it naturally explains the orbital parameters of the satellites as well as some features observed on Mars such as (i) its excess of prograde angular momentum possibly caused by a large impact (Craddock 2011), and (ii) the existence of a large population of oblique impact craters at its surface that may record the slow orbital decay of ancient moonlets formed from the impact-generated accretion disk (Schultz & Lutz-Garihan 1982). Along these lines, Citron et al. (2015) have recently shown that a large impact (impactor with 0.01–0.02 Mars masses) would generate a circum-Mars debris disk comprising ∼1–4% of the impactor mass, thus containing enough mass to form both Phobos and Deimos. Although the impact scenario has become really attractive, it has not yet been demonstrated that it can explain the physical properties and spectral characteristics of the martian moons. Here, we investigate the mineralogical composition and texture of the dust that would have crystallized in an impact-generated accretion disk. Since there are no firm constraints regarding the thermodynamic properties of the disk, we perform our investigation for various thermodynamic conditions and impactor compositions. We show that under specific disk’s pressure and temperature conditions, Phobos and Deimos’ physical and orbital properties can be finally reconciled. 2. FORMATION FROM A COOLING MAGMA
Because of the absence of constraints regarding the composition (Mars-dominated or impactor-dominated) and the thermodynamic conditions of the impact-
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generated disk, several configurations must be investigated in order to understand the formation conditions of Phobos and Deimos within such a scenario. As a first step, we considered the protolunar disk as a reference case because it is so far the most studied impactgenerated accretion disk. Its structure has been investigated by Thompson & Stevenson (1988) and subsequently by Ward (2012, 2014). These studies have shown that the disk’s midplane consists of a liquid phase surrounded by a vapor atmosphere. Beyond the Roche limit, gravitational instabilities developed and large clumps formed directly from the magma (e.g. Salmon & Canup 2012; Kokubo et al. 2000). Those clump then agglomerated to form the Moon. In this case, because of internal evolutionary processes (differentiation, convection, etc..), the mineralogical composition and thus the spectral properties of the lunar mantle and crust will differ from the clump’ ones. In the martian case, the situation is different in the sense that the clumps possess right away a mass/size comparable to that of Phobos and Deimos (Rosenblatt & Charnoz 2012). This implies that the final composition and spectral properties of the martian moons would directly reflect those of the minerals that crystallized from the magma disk. 2.1. Methods
We considered three different compositions for the impactor (see Table 1), namely a Mars-like composition (1), a Moon-like composition (2) and an outer solar system composition (3) (i.e., TNO). The latter case would be coherent with an inward migration of a large planetesimal as a consequence of the possible late migration of the giant planets (e.g., Morbidelli et al. 2005). By analogy with the Earth-Moon system, it has been suggested, however, that the impactor most probably formed near the proto-Mars (Hartmann & Davis 1975, see cases (1) and (2)) but one cannot exclude that the impactor formed elsewhere (3). In addition, since the relative proportions of the impactor and martian materials are poorly constrained in the resulting disk, we considered various proportions between these two materials. We considered two cases, namely a disk exclusively made of the impactor mantle and a half–half fraction. Case (1) complements this sequence by illustrating the case for a 100% fraction of the martian mantle. To estimate the composition of the solids crystallized from the magma and thus of the moons, we performed a CIPW normative mineralogy calculation (GonzalezGuzman 2016). This method allows determining the nature of the most abundant minerals that crystallize from an anhydrous melt at low pressure while providing at the same time a good estimate of their final proportions. The CIPW norm calculation is well adapted to our case given that the disk supposedly cooled down slowly through radiation (Ward 2012), allowing complete crystallization of the minerals. It should be noted that we do not aim at determining the exact composition of the moons. Considering the few constraints we have on the system, our purpose is to discriminate between plausible and unplausible scenarios and thus provide new constraints for future studies. 2.2. Results
In this section, we present the inferred mineralogical composition of the moons for the three aforementioned impactor compositions (see Table 2) and for the different relative abundances of the impactor and martian mantle. • Case 1 (Mars-like impactor) : Since the impactor has a composition similar to that of Mars, we performed the calculation using a Bulk Silicate Mars (BSM) magma composition (taken from Lodders & Fegley 2011). The BSM is an estimate of the chemical composition of Mars’ mantle. By calculating the CIPW norm, we found that both olivine and orthopyroxene (hypersthene) are the main minerals to crystallize (∼59% and ∼21% respectively). Both diopside and feldspar (plagioclase) are also formed although in significantly lower proportions (∼7% and ∼12% respectively). Note, however, that the above results do not account for a partial vaporization of the disk. The fraction of vaporized material is speculative although theoretical considerations advocate that it should be more than 10% in the case of the protolunar disk (Ward 2012, 2014). To emphasize the role of vaporization on the resulting composition of the building blocks of the moons, we considered the case of a half vaporized disk (see Table 1b). Its magma composition was derived following the results of Canup et al. (2015) for a Bulk Silicate Earth (BSE) disk’s composition. This first order approximation is quantitatively valid as the BSE and a BSM compositions are very similar Visscher & Fegley (2013). By applying the CIPW norm to this new magma composition, we found that significantly more olivine is crystallized (∼85%), whereas both orthopyroxene and diopside do not form. The proportion of feldspar remains, however, the same (∼10%). • Case 2 (Moon-like impactor) : Here, we used the Bulk Silicate Moon composition as a proxy for the impactor composition. For both a 50-50% MoonMars mixing ratio and a pure lunar-like composition, we found that both olivine and orthopyroxene (hypersthene) are the main crystallizing minerals (∼60% and ∼22% respectively). In both cases, it thus appears that the derived bulk composition of the moons is very close to the one obtained for a Bulk Silicate Mars disk’s composition. Taking into account a partial vaporization of the magma would also lead to results similar to those obtained for case 1. • Case 3 (TNO-like impactor) : Here we used the composition of interplanetary dust particles (IDPs; Rietmeijer 2009) as a proxy for the composition of the TNO-like impactor. IDPs which are the likely building blocks of comets may also be the ones of TNOs if one follows the basic and currently accepted assumption that both population formed in the outer solar system. However, by using directly the composition of IDP grains, we neglect the effect of differentiation that has likely occurred on a Moon-sized TNO. This implies that we certainly overestimate the amount of iron in the disk.
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New constraints on the nature of the martian moons’ building blocks
(a) Intact capture
B
Others 12%
S
Mass fraction of the different asteroid spectral types
8%
19%
32%
C
5%
D
24%
P Normalized reflectance
(b) Co-accretion
Reddened ordinary chondrites 3.0 Reddened angrite Enstatite chondrite-like asteroids 2.5 Deimos Phobos Red 2.0 1.5 1.0 0.5
1.0
1.5 Wavelength (µm)
Normalized reflectance
2.5
Gas to solids : submicron-sized grains
Magma to solids : micron-sized grains
(c) Large impact
2.0
Lunar mare (20-45 µm) 3.0 Deimos Phobos Red
D-type asteroids (
